For transmitting data, it is these days a prominent solution, to rely on optical data transmission. For transmitting data bits, one or more symbols may be transmitted by means of an optical transmission signal, wherein a data symbol represents a set of data bits.
A symbol, alternatively called data symbol, is transmitted, by modulating the phase and/or the amplitude of an optical carrier signal in dependence on the set of data bits which shall be represented by this symbol. The modulation of the optical carrier signal is carried out in accordance with a constellation scheme of a respective modulation format.
One prominent candidate of a modulation format relying on phase modulation is that of binary phase-shift keying (BPSK). In BPSK, the phase of the optical carrier signal is modulated, such that it takes on one out of two discrete phase values. Thus, in BPSK, a symbol represents one data bit.
Another prominent candidate of a modulation format carrying out phase modulation is that of quaternary phase-shift keying (QPSK), which may be called alternatively four quadrature amplitude modulation (4QAM). In QPSK, the phase of the optical carrier signal is modulated, such that it takes on one out of four discrete phase values. Thus, in QPSK, a symbol represents two data bits that are to be transmitted.
Another prominent candidate of a modulation format is 8QAM, in which the phase and the amplitude of the optical carrier signal are modulated in dependence on the set of data bits, that shall be represented by data symbols. The phase and the amplitude of the optical carrier signals are modulated, such that they take on one out of eight discrete values of the corresponding constellation scheme. Thus, in 8QAM a symbol represents three data bits.